I. Field of the Invention
The present invention relates to techniques for semiconductor processing, and more particularly to semiconductor processing which may be performed at low temperatures.
II. Description of the Related Art
In the field of semiconductor processing, there have been several attempts to use lasers to convert thin amorphous silicon films into polycrystalline films. For example, in James Im et al., xe2x80x9cCrystalline Si Films for Integrated Active-Matrix Liquid-Crystal Displays,xe2x80x9d 11 MRS Bullitin 39 (1996), an overview of conventional excimer laser annealing technology is presented. In such a system, an excimer laser beam is shaped into a long beam which is typically up to 30 cm long and 500 microns or greater in width. The shaped beam is scanned over a sample of amorphous silicon to facilitate melting thereof and the formation of polycrystalline silicon upon resolidification of the sample.
The use of conventional excimer laser annealing technology to generate polycrystalline silicon is problematic for several reasons. First, the polycrystalline silicon generated in the process is typically small grained, of a random microstructure, and having a nonuniform grain sizes, therefore resulting in poor and nonuniform devices and accordingly, low manufacturing yield. Second, in order to obtain acceptable performance levels, the manufacturing throughput for producing polycrystalline silicon must be kept low. Also, the process generally requires a controlled atmosphere and preheating of the amorphous silicon sample, which leads to a reduction in throughput rates. Accordingly, there exists a need in the field to generate higher quality polycrystalline silicon at greater throughput rates.
An object of the present invention is to provide techniques for growing large grained polycrystalline or single crystal silicon structures using energy-controllable laser pulses.
A further object of the present invention is to utilize small-scale translation of a silicon sample in order to grow large grained polycrystalline or single crystal silicon structures on the sample.
Yet another object of the present invention is to provide techniques for growing location controlled large grained polycrystalline or single crystal silicon structures which yield planarized thin silicon films.
Yet a further object of the present invention is to provide techniques for growing large grained polycrystalline or single crystal silicon structures at low temperatures, for example at room temperature, and without preheating.
A still further object of the present invention is to provide techniques for coordinated attenuation of laser fluence.
In order to achieve these objectives as well as others that will become apparent with reference to the following specification, the present invention provides an excimer laser for generating a plurality of excimer laser pulses of a predetermined fluence, an energy density modulator for controllably modulating fluence of the excimer laser pulses, a beam homoginizer for homoginizing modulated laser pulses in a predetermined plane, a mask for masking portions of the homoginized modulated laser pulses into patterned beamlets, a sample stage for receiving the patterned beamlets to effect melting of portions of any amorphous silicon thin film sample placed thereon corresponding to the beamlets, translating means for controllably translating a relative position of the sample stage with respect to a position of the mask and a computer for controlling the controllable fluence modulation of the excimer laser pulses and the controllable relative positions of the sample stage and mask, and for coordinating excimer pulse generation and fluence modulation with the relative positions of the sample stage and mask, to thereby process amorphous silicon thin film sample into a single or polycrystalline silicon thin film by sequential translation of the sample stage relative to the mask and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.
In a preferred arrangement, the excimer laser is a ultraviolet excimer laser, and the energy density modulator includes a rotatable wheel, two or more beam attenuators circumferentially mounted on the wheel, and a motor, for controllably rotating the wheel such that each sequential pulse emitted by the laser passed through one of the two or more beam attenuators. Advantageously, the beam attenuators are capable of producing at least two different levels of fluence attenuation.
In an alternative arrangement, the energy density modulator includes a multilayer dialectic plate that is rotatable about an axis perpendicular to a path formed by the excimer pulses, in order to variably fluence modulate the excimer pulses in dependance of an angle formed between the excimer pulse path and the axis of rotation. A compensating plate is advantageously provided to compensate for a dialectic induced shift in the beam path,
In another alternative arrangement, the energy density modulator includes one or more beam attenuators and a translating stage for controllably translating the one or more beam attenuators such that each sequential pulse emitted by the laser passes through one of the one or more beam attenuators or passes through the energy density modulator without passing through any of the one or more beam attenuators. The translating stage is movable in both a direction parallel to a path formed by the excimer pulses and a direction perpendicular to the path, and the beam attenuators are positionable such that the excimer pulses pass through one of the one or more beam attenuators or through none of the one or more beam attenuators.
In still another alternative arrangement, the energy density modulator includes one or more movable beam attenuators being controllably moved such that each sequential pulse emitted by the laser passes through one or more of the one or more beam attenuators or passes through the energy density modulator without passing through any of the one or more beam attenuators.
In one preferred arrangement, the translating means includes a mask translating stage that is translatable in both orthogonal directions that are perpendicular to a path formed by the homoginized beams, and a translating stage motor for controllably translating the mask translating stage in both of the translatable directions under control of the computer. In an alternative arrangement, the translating means includes the sample translating stage, and has an X direction translation portion and a Y direction translation portion permitting movement in two orthogonal directions that are perpendicular to a path formed by the patterned beamlets and being controllable by the computer for controllably translating the sample in both of the translatable directions under control of the computer. The sample translation stage may additionally include a Z direction translation portion, for permitting movement of the sample in a direction parallel to the path formed by the patterned beamlets. Most preferably, the entire system is mounted on a granite block to stabilizing the sample from ambient vibration.
The present invention also provides methods for processing an amorphous silicon thin film sample into a single or polycrystalline silicon thin film. In a preferred technique, the method includes the steps of generating a plurality of excimer laser pulses of a predetermined fluence; controllably modulating the fluence of the excimer laser pulses; homoginizing the modulated laser pulses in a predetermined plane; masking portions of the homoginized modulated laser pulses into patterned beamlets, irradiating an amorphous silicon thin film sample with the patterned beamlets to effect melting of portions thereof corresponding to the beamlets; and controllably translating the sample with respect to the patterned beamlets and with respect to the controlled modulation to thereby process the amorphous silicon thin film sample into a single or polycrystalline. silicon thin film by sequential translation of the sample relative to the patterned beamlets and irradiation of the sample by patterned beamlets of varying fluence at corresponding sequential locations thereon.